ref: 1db2e7b5b470a47da9ca237ad704c2b73f29af2e
dir: /engine/overworld/sprite_collisions.asm/
_UpdateSprites:: ld h, $c1 inc h ld a, wSpritePlayerStateData2ImageBaseOffset - wSpritePlayerStateData2 .spriteLoop ld l, a sub wSpritePlayerStateData2ImageBaseOffset - wSpritePlayerStateData2 ld c, a ld [hCurrentSpriteOffset], a ld a, [hl] and a jr z, .skipSprite ; tests $c2Xe push hl push de push bc call .updateCurrentSprite pop bc pop de pop hl .skipSprite ld a, l add $10 ; move to next sprite cp wSpritePlayerStateData2ImageBaseOffset - wSpritePlayerStateData2 ; test for overflow (back at beginning) jr nz, .spriteLoop ret .updateCurrentSprite cp $1 jp nz, UpdateNonPlayerSprite jp UpdatePlayerSprite UpdateNonPlayerSprite: dec a swap a ld [hTilePlayerStandingOn], a ; $10 * sprite# ld a, [wNPCMovementScriptSpriteOffset] ; some sprite offset? ld b, a ld a, [hCurrentSpriteOffset] cp b jr nz, .unequal jp DoScriptedNPCMovement .unequal jp UpdateNPCSprite ; This detects if the current sprite (whose offset is at hCurrentSpriteOffset) ; is going to collide with another sprite by looping over the other sprites. ; The current sprite's offset will be labelled with i (e.g. $c1i0). ; The loop sprite's offset will labelled with j (e.g. $c1j0). ; ; Note that the Y coordinate of the sprite (in [$c1k4]) is one of the following ; 9 values when the sprite is aligned with the grid: $fc, $0c, $1c, $2c, ..., $7c. ; The reason that 4 is added below to the coordinate is to make it align with a ; multiple of $10 to make comparisons easier. DetectCollisionBetweenSprites: nop ld h, wSpriteStateData1 / $100 ld a, [hCurrentSpriteOffset] add wSpriteStateData1 % $100 ld l, a ld a, [hl] ; a = [$c1i0] (picture) (0 if slot is unused) and a ; is this sprite slot slot used? ret z ; return if not used ld a, l add 3 ld l, a ld a, [hli] ; a = [$c1i3] (delta Y) (-1, 0, or 1) call SetSpriteCollisionValues ld a, [hli] ; a = [$C1i4] (Y screen coordinate) add 4 ; align with multiple of $10 ; The effect of the following 3 lines is to ; add 7 to a if moving south or ; subtract 7 from a if moving north. add b and $f0 or c ld [hFF90], a ; store Y coordinate adjusted for direction of movement ld a, [hli] ; a = [$c1i5] (delta X) (-1, 0, or 1) call SetSpriteCollisionValues ld a, [hl] ; a = [$C1i6] (X screen coordinate) ; The effect of the following 3 lines is to ; add 7 to a if moving east or ; subtract 7 from a if moving west. add b and $f0 or c ld [hFF91], a ; store X coordinate adjusted for direction of movement ld a, l add 7 ld l, a xor a ld [hld], a ; zero [$c1id] XXX what's [$c1id] for? ld [hld], a ; zero [$c1ic] (directions in which collisions occurred) ld a, [hFF91] ld [hld], a ; [$c1ib] = adjusted X coordinate ld a, [hFF90] ld [hl], a ; [$c1ia] = adjusted Y coordinate xor a ; zero the loop counter .loop ld [hFF8F], a ; store loop counter swap a ld e, a ld a, [hCurrentSpriteOffset] cp e ; does the loop sprite match the current sprite? jp z, .next ; go to the next sprite if they match ld d, h ld a, [de] ; a = [$c1j0] (picture) (0 if slot is unused) and a ; is this sprite slot slot used? jp z, .next ; go the next sprite if not used inc e inc e ld a, [de] ; a = [$c1j2] ($ff means the sprite is offscreen) inc a jp z, .next ; go the next sprite if offscreen ld a, [hCurrentSpriteOffset] add 10 ld l, a inc e ld a, [de] ; a = [$c1j3] (delta Y) call SetSpriteCollisionValues inc e ld a, [de] ; a = [$C1j4] (Y screen coordinate) add 4 ; align with multiple of $10 ; The effect of the following 3 lines is to ; add 7 to a if moving south or ; subtract 7 from a if moving north. add b and $f0 or c sub [hl] ; subtract the adjusted Y coordinate of sprite i ([$c1ia]) from that of sprite j ; calculate the absolute value of the difference to get the distance jr nc, .noCarry1 cpl inc a .noCarry1 ld [hFF90], a ; store the distance between the two sprites' adjusted Y values ; Use the carry flag set by the above subtraction to determine which sprite's ; Y coordinate is larger. This information is used later to set [$c1ic], ; which stores which direction the collision occurred in. ; The following 5 lines set the lowest 2 bits of c, which are later shifted left by 2. ; If sprite i's Y is larger, set lowest 2 bits of c to 10. ; If sprite j's Y is larger or both are equal, set lowest 2 bits of c to 01. push af rl c pop af ccf rl c ; If sprite i's delta Y is 0, then b = 7, else b = 9. ld b, 7 ld a, [hl] ; a = [$c1ia] (adjusted Y coordinate) and $f jr z, .next1 ld b, 9 .next1 ld a, [hFF90] ; a = distance between adjusted Y coordinates sub b ld [hFF92], a ; store distance adjusted using sprite i's direction ld a, b ld [hFF90], a ; store 7 or 9 depending on sprite i's delta Y jr c, .checkXDistance ; If sprite j's delta Y is 0, then b = 7, else b = 9. ld b, 7 dec e ld a, [de] ; a = [$c1j3] (delta Y) inc e and a jr z, .next2 ld b, 9 .next2 ld a, [hFF92] ; a = distance adjusted using sprite i's direction sub b ; adjust distance using sprite j's direction jr z, .checkXDistance jr nc, .next ; go to next sprite if distance is still positive after both adjustments .checkXDistance inc e inc l ld a, [de] ; a = [$c1j5] (delta X) push bc call SetSpriteCollisionValues inc e ld a, [de] ; a = [$c1j6] (X screen coordinate) ; The effect of the following 3 lines is to ; add 7 to a if moving east or ; subtract 7 from a if moving west. add b and $f0 or c pop bc sub [hl] ; subtract the adjusted X coordinate of sprite i ([$c1ib]) from that of sprite j ; calculate the absolute value of the difference to get the distance jr nc, .noCarry2 cpl inc a .noCarry2 ld [hFF91], a ; store the distance between the two sprites' adjusted X values ; Use the carry flag set by the above subtraction to determine which sprite's ; X coordinate is larger. This information is used later to set [$c1ic], ; which stores which direction the collision occurred in. ; The following 5 lines set the lowest 2 bits of c. ; If sprite i's X is larger, set lowest 2 bits of c to 10. ; If sprite j's X is larger or both are equal, set lowest 2 bits of c to 01. push af rl c pop af ccf rl c ; If sprite i's delta X is 0, then b = 7, else b = 9. ld b, 7 ld a, [hl] ; a = [$c1ib] (adjusted X coordinate) and $f jr z, .next3 ld b, 9 .next3 ld a, [hFF91] ; a = distance between adjusted X coordinates sub b ld [hFF92], a ; store distance adjusted using sprite i's direction ld a, b ld [hFF91], a ; store 7 or 9 depending on sprite i's delta X jr c, .collision ; If sprite j's delta X is 0, then b = 7, else b = 9. ld b, 7 dec e ld a, [de] ; a = [$c1j5] (delta X) inc e and a jr z, .next4 ld b, 9 .next4 ld a, [hFF92] ; a = distance adjusted using sprite i's direction sub b ; adjust distance using sprite j's direction jr z, .collision jr nc, .next ; go to next sprite if distance is still positive after both adjustments .collision ld a, [hFF91] ; a = 7 or 9 depending on sprite i's delta X ld b, a ld a, [hFF90] ; a = 7 or 9 depending on sprite i's delta Y inc l ; If delta X isn't 0 and delta Y is 0, then b = %0011, else b = %1100. ; (note that normally if delta X isn't 0, then delta Y must be 0 and vice versa) cp b jr c, .next5 ld b, %1100 jr .next6 .next5 ld b, %0011 .next6 ld a, c ; c has 2 bits set (one of bits 0-1 is set for the X axis and one of bits 2-3 for the Y axis) and b ; we select either the bit in bits 0-1 or bits 2-3 based on the calculation immediately above or [hl] ; or with existing collision direction bits in [$c1ic] ld [hl], a ; store new value ld a, c ; useless code because a is overwritten before being used again ; set bit in [$c1ie] or [$c1if] to indicate which sprite the collision occurred with inc l inc l ld a, [hFF8F] ; a = loop counter ld de, SpriteCollisionBitTable add a add e ld e, a jr nc, .noCarry3 inc d .noCarry3 ld a, [de] or [hl] ld [hli], a inc de ld a, [de] or [hl] ld [hl], a .next ld a, [hFF8F] ; a = loop counter inc a cp $10 jp nz, .loop ret ; takes delta X or delta Y in a ; b = delta X/Y ; c = 0 if delta X/Y is 0 ; c = 7 if delta X/Y is 1 ; c = 9 if delta X/Y is -1 SetSpriteCollisionValues: and a ld b, 0 ld c, 0 jr z, .done ld c, 9 cp -1 jr z, .ok ld c, 7 ld a, 0 .ok ld b, a .done ret SpriteCollisionBitTable: db %00000000,%00000001 db %00000000,%00000010 db %00000000,%00000100 db %00000000,%00001000 db %00000000,%00010000 db %00000000,%00100000 db %00000000,%01000000 db %00000000,%10000000 db %00000001,%00000000 db %00000010,%00000000 db %00000100,%00000000 db %00001000,%00000000 db %00010000,%00000000 db %00100000,%00000000 db %01000000,%00000000 db %10000000,%00000000